Emulate gets called from dos.c. It initializes DOSEMU to
prepare it for running in vm86 mode. This involves catching signals,
preparing memory, calling all the initialization functions for the I/O
subsystems (video/serial/etc...), getting the boot sector instructions
and calling vm86().

DOSEMU must not work within the 1 meg DOS limit, so
start of code is loaded at a higher address, at some time this could
conflict with other shared libs. If DOSEMU is compiled statically
(without shared libs), and org instruction is used to provide the jump
above 1 meg.

The `vm86_struct` is used to pass all the necessary status/registers to
DOSEMU when running in vm86 mode.

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DOSEMU keeps system wide configuration status in a structure
called config.

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The var `fatalerr` can be given a true value at any time to have DOSEMU
exit on the next return from vm86 mode.

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The var 'running_DosC' is set by the DosC kernel and is used to handle
some things differently, e.g. the redirector.
It interfaces via INTe6,0xDC (DOS_HELPER_DOSC), but only if running_DosC
is !=0. At the very startup DosC issues a INTe6,0xdcDC to set running_DosC
with the contents of BX (which is the internal DosC version).